A tunable laser is very useful in the field of spectrometry for various materials and optical elements, and measurement for environment and medical treatment.
The tunable external resonator laser is one of the tunable lasers.
The tunable external resonator laser has an optical gain medium such as semiconductor laser chip, and has an optical element which has a frequency discrimination function such as a diffraction grating. The tunable external resonator laser can freely control the oscillation wavelength.
Specially, the tunable external resonator laser using a gain medium, such as a laser diode, and can oscillate light with a wide wavelength range. Further, it has an advantage that it can be made small, strong and relatively inexpensive.
Namely, the tunable external resonator laser can be used for many applications.
There are many kinds of tunable external resonator lasers.
Specially, the mode-hop-free laser which can continuously control the oscillation wavelength is very important for spectrometry because accurate spectrometry can be achieved by suppressing the mode-hop.
Furthermore, mode-hop free tuning is necessary in the case of an automatic calibration for the oscillation wavelength of the tunable resonator laser using the Fabry-Perot resonance pattern, such as a solid etalon, and locking the oscillating frequency of the tunable resonator laser.
A Littrow-type tunable external resonator laser and a Littman-type tunable external resonator laser are known as mode-hop-free lasers.
The Littman-type tunable external resonator laser comprises a diffraction grating like the Littrow-type tunable external resonator laser. However, the Littrow-type rotates a diffraction grating against a laser beam, on the other hand, in the Littman-type tunable external resonator laser, the diffraction grating is fixed against the laser beam.
Therefore, it is possible to secure the area of the laser light which is incident on the diffraction grating, and to realize a small and high-resolution resonator arrangement.
As a result, it is widely used as a tunable resonator laser because it has an advantage that the emission direction of the output light is not changed, even if the wavelength is changed.
Non-Patent Document 1 disclose a Littrow-type tunable external resonator laser.
Furthermore, Non-Patent Document 2, Patent Document 1, 2 and 3 discloses a Littman-type tunable external resonator laser.
A Littman-type tunable external resonator laser is a typical tunable external resonator laser because it is possible to tune a wavelength widely and continuously, and suppresses the power variation of the output light, even if wavelength tuning is performed.